1. Field of the Invention
This invention relates to the field of tongs, of the type commonly used for making up, and breaking apart, threaded connection between tubular member and the like.
2. Description of the Prior Art
Four categories of threaded joint tubular members, or rod elements are used in well drilling and production: casing, drill pipe, tubing and sucker rods. Available in incremental lengths, these members must be successively joined and lowered into the well or, conversely, separated and removed therefrom. Joint sections generally are circular, and the tubulars have no provision for keyed type engagement with a tong mechanism. The joint grip mechanism of a tong translates a rotative input force into coplanar vector forces, acting chordally across the joint section. At the points of grip contact with the joint surface, these chordal forces are resolved into normal and tangential components relative to the joint contour. Consequently, the rotative torque delivery capability of the tong system is a function of the normal component of the chordal vector multiplied by the coefficient of drag at the joint contact points of the grip elements.
The coefficient of drag is similar to the coefficient of friction, but includes the effects of friction plus shear plane interferences incident to surface irregularities and the impression of the grip element into the joint surface. Grip elements can be provided with multiple serrations, or penetration features, to provide the interference contact needed at the joint surface for the development of a suitably high coefficient of drag. Since torque delivery capability is a function of the normal force time the drag coefficient times the radius of the joint to be worked, the required magnitude of the normal force varies inversely with the coefficient of drag developed at the contact between the grip elements and the joint surface. The progressive refinement of tubular installation procedures and use practices, has mandated limitation and control of grip element penetrations into the joint surface. Consequently, the distribution and balance of grip element energizing forces are critical factors in the design, development and evaluation of a tong mechanism; while the effective diametrical range of grip capability is limited by the slope of the actuating force vector and the coefficient of drag at grip points.
Various mechanisms involving linkages, levers, wedges, and cams are in current use for the disposition and balance of the normal and tangential force components. Usually, grip elements, or dies, are arcuately, disposed within carrier bodies, or jaws, which span a circumferential segment of the joint surface. These jaws are structured to accept the translated input chordal vector and delivery it to the joint surface in normal and tangential components. A degree or comprise must be established to accommodate acceptable ranges of joint and mechanism dimensional tolerance.
Design compromises, common to the art, structure jaws to operate with very high load variations between leading and trailing dies, or resort to jaw guiding slides, or linkages, to control die contact and tangential force delivery ratios. However, all jaw guides absorb energy and detract from torque delivery. Also, extremely uneven die loading causes excessive marring, or damage, to the joint surface.
A very prevalent compromise in tong design is the continuation of a counter-reactive brake force after jaw engagement has been established. This is necessary for those linkage types that are not self-energizing; that is, those that will not automatically respond to increase of input force by tightening grip on the joint. The net result of the brake action is a proportionate reduction of the torque delivered.
Prior art discloses two elementary mechanisms for the translation of rotative force from a torque input member to a joint surface, namely:
1. The pivoted solid bar, as shown diagrammatically in FIG. 3, and disclosed in practice in the U.S. Pat. No. 1,811,666.
2. The solid bar in combination with a cam track connection to the torque input member, as disclosed in U.S. Pat. No. 3,023,651 and shown diagrammatically in FIG. 6. In the development of the art, various supplementary positioning and guiding options have been applied in an attempt to enhance the effectiveness of the grip systems. Each of these elementary mechanisms represents design compromises in the prior art which detract from tong effectiveness.
The pivoted solid bar mechanism requires an arcuate surface so that its camming action is effective; but since the surface is arcuate, there is line contact with the joint surface. Line contact with the joint surface increases the hazard of joint scoring or crushing due to the contact pressure developed.
The second elementary mechanism is the combination of a solid bar having multiple joint engaging elements and a cam follower which follows a cam trak in the tong. Characteristically, the developed contact force will be uneven from contact point to contact point; i.e., the cam track mechanism spreads grip element die contact circumferentially on the joint, but tends to load the forwad die increasingly as the link or bar elements tends to roll with the rotary force.
Various systems utilizing all or parts of these basic mechanisms are currently in use. For example, the Hillman-Kelly tong disclosed in U.S. Pat. No. 2703221, having a pivotally fixed jaw and a jaw actuated by a lever element which is pivotally and stationarily connected to a draw head. In the Weatherford GMBH tong disclosed in U.S. Pat. No. 4,192,206, a lever element pivotally connected to a carrier member receives input force through a cam section, the lever element having pivotally connected thereto a rocker jaw.
There are also in common usage various modifications of these basic mechanisms. All of these systems are subject to the same limitations and range considerations as those of the basic mechanisms.